Rub both sides of flank steak with taco seasoning mix. In 12-inch nonstick skillet, heat oil over medium-high heat until hot. Cook steak in oil 8 minutes, turning once. Decrease heat to medium; cover skillet. Cook 15 to 20 minutes or until meat thermometer inserted in thickest part of the steak reads 165°F. Let stand 3 minutes. Thinly slice across the grain.
Heat tortillas as directed on package. Fill tortillas with flank steak, chiles, avocado and sour cream.